This invention relates to the preparation of highly densified ceramic bodies by the pyrolysis of a mixture comprising a preceramic borosiloxane, silicon carbide powder, a curing agent for the borosiloxane and, optionally, a crosslinking agent for the borosiloxane and/or additional components to facilitate sintering. Such highly densified ceramic bodies can be prepared by sintering under pressure or by utilizing a pressureless process.
This invention also relates to novel mixtures comprising a preceramic borosiloxane, silicon carbide powder, a curing agent for the borosiloxane and, optionally, a crosslinking agent for the borosiloxane and/or additional components to facilitate sintering. These mixtures can be formed into desired shapes and then sintered to form ceramic, shaped bodies with high densities.
The products and processes of the present invention have several distinct advantages over prior art methods. First, the green bodies have high strengths, thus, facilitating handling and machining before sintering. Secondly, the use of a borosiloxane often eliminates the need to include additional sintering aids (e.g. boron or aluminum) and/or carbon sources into the preceramic mixture since both may be incorporated into the polymeric structure of said borosiloxane. Finally, the composition of the preceramic mixture can be varied to accommodate various molding techniques such as press and sinter or transfer/injection molding and sinter applications.
Prockazka in U.S. Pat. Nos. 4,004,934 and 4,041,117 and Prockazka et al. in U.S. Pat. No. 4,081,284 describe high density silicon carbide ceramic bodies prepared in a pressureless sintering process. Such ceramic bodies were prepared from mixtures consisting of silicon carbide powder, a boron-containing additive, and a carbonaceous additive. The carbonaceous additives or binders are high molecular weight, carbon-based aromatic compounds such as phenol-formaldehyde condensate-novolak, resorcinol-formaldehyde, aniline-formaldehyde, cresol-formaldehyde, derivatives of polynuclear aromatic hydrocarbon compounds contained in coal tar, polyphenylene, polymethylphenylene, and the like. Ceramic silicon carbide materials with greater than 85% of theoretical density were obtained.
Onda et al. in Japanese Kokai Patent 60-16869 describe a method of preparing high density silicon carbide ceramics which involves pressureless sintering of a mixture consisting of silicon carbide powder, an auxiliary sintering aid, and an organosilicon polymer characterized by a Si--C skeletal structure. Disclosed sintering aids include boron, aluminum, and beryllium containing compounds. The organosilicon polymers were described as "high-molecular-weight organic silicon-containing compounds with silicon-carbon skeletal structures." The organosilicon polymers were not further identified even in the examples. Based on this limited description it appears likely that the organosilicon polymers were polycarbosilanes which have a Si--C skeletal backbone.
Yajima et al. reported the formation of silicon carbide sintered bodies from borosiloxanes in Nature, vol. 266, pp 522-524. The process disclosed therein comprises blending a borosiloxane and silicon carbide powder in benzene, evaporating the solvent, mould pressing the mixture and sintering at a temperature up to 1700.degree. C. This reference, however, does not achieve a high density product as the temperature utilized therein is less than 1900.degree. C. Moreover, since the reference does not teach the use of a curing or crosslinking agent, green bodies formed thereby are generally not as strong as those formed by the process of the present invention.
Several inventors such as Tamamizu et al. in Japanese Kokai Patent 54-134744 and Yajima et al. in J. Am. Ceram Soc. Bull., Vol. 60, p 253 have reported the use of a mixture of silicon carbide powder and a borosiloxane polymer as a bonding agent. These references, however, as in Yajima et al. supra, do not achieve high density materials because of the temperature utilized, and generally do not form strong green bodies as a curing agent and cross-linking agent are not included.
In copending applications, Ser. Nos. 07/248,884, 07/249,036, 07/265,988 and 07/268,398 in the names of Atwell et al. or Burns et al. and assigned to the same assignee hereof, there are disclosed methods of producing high density silicon carbide sintered bodies by forming a mixture of polysilazanes, polysilanes or polysiloxanes, metal-containing sintering aids, optional curing agents and silicon carbide powders into green bodies and then sintering them in an inert atmosphere at temperatures in excess of 1900.degree. C.
The present application discloses for the first time that high density, high strength ceramic products may unexpectedly be obtained by sintering a mixture comprising a preceramic borosiloxane, silicon carbide powder, a curing agent for the borosiloxane and, optionally, a crosslinking agent for the borosiloxane and/or additional components to facilitate sintering.